Data Transfer Rate
Data transfer rate (or often just data rate) is the amount of digital data that moved from one place to another in a second's time. Data rates have advanced dramatically in the last 10 years. SCSI data rates have gone from 5 million bytes (Mbytes) per second to 320 Mbytes per second. While disk based products today often specify and, under certain conditions can achieve, data rates in the 100 Mbytes per second range, in transaction processing, the limitations imposed by rotating disk latency reduce the actual data rate delivered to the server by several orders of magnitude.
Data Rate in High Transaction Rate Environments
Operating systems and databases break up accesses to disk into relatively small amounts of data called extents. The common extent size for a UNIX system is 8192 Bytes (8kBytes). For an NT system, the extent size is 4096 Bytes (4kBytes). Databases also use extents of these magnitudes. An extent of this size contains a significant amount of data. It is large enough to contain the information in all but the most sophisticated of transactions. At 100 Mbytes per second, one 4k Byte extent would be delivered in .00004 seconds, 40 microseconds. However, if the disk heads are required to move, no data will begin to flow on average until the .01-second disk drive latency has elapsed. When transactions are random and heads move for each transaction, the 100Mbyte per second rotating disk delivers only 400 Kbytes of data per second. Note that this is but 100 transactions per second and only .4% of the "specified" data rate.
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